Normally I euthanize by putting the fish under with clove oil (full strength, also sold as eugenol), letting it sit until the gills don't move after a full minute, then severing the spinal cord (right behind the head) with a knife. It's bloody, but it makes sure the fish is gone. (I heard very bad stories of fish not dying under clove oil alone) The fish do appear agitated as they go under, but this can be solved by slowly working up the clove oil levels instead of dropping the fish in full strength.
I've also heard of putting the fish in a bag, then slamming a rock or brick onto the fish, fast enough so the fish is gone in a moment. I haven't preformed this method, both because I haven't had the chance, and because I'm a little nervous to squish them. Just my own preference.
As a disclaimer, I don't know the exact levels of pain or anxiety that the fish feels during these. In my opinion, we can only try to make the fish as comfortable as we can, and the death quick.
(I really hope this reply isn't too graphic)